1. Field of the Invention
The present invention relates to an exposure apparatus used for the manufacture of a semiconductor device, and more particularly to a cata-dioptric optical system for reduction-projecting an enlarged real element pattern on a reticle.
2. Related Background Art
Semiconductor integrated circuit have been becoming finer and finer, and a higher resolution power is required for an exposure apparatus for making a pattern. In order to meet the requirement, it is necessary to shorten a wavelength of a light source and increase a numerical aperture (N.A.) of an optical system. However, as the wavelength is shortened, a glass material which can be practically used is limited due to the light absorption.
When the wavelength is shorter than 300 nm, the practically usable glass materials are only synthesized quartz and fluorspar (calcium fluoride). However, because of a poor temperature characteristic of the fluorspar, it may be used in only a small portion of optical elements of the optical system, and it is not desirable to use it in quantity. In addition, it is very difficult to manufacture a projection optical system which meets the above requirement by only a refraction system. It is almost impossible to manufacture a projection optical system having a large numerical of aperture by only a reflection system, because of difficulty in aberration correction.
Accordingly, various techniques to form the projection optical system by combining a reflection system and a refraction system have been proposed. As one example thereof, a ring view field optical system is disclosed in U.S. Pat. No. 4,747,678. In this optical system, off-axis light beams are used to present interference between an incident light and a reflected light, and only off-axis ring band portions are exposure areas. As a result, it is difficult to increase the numerical of aperture. Further, since it is not possible to expose an entire wafer to light when a pattern on a reticle or mask is to be transferred to the wafer, it is necessary to make the exposure while the reticle and the wafer are moved at different velocities determined by a reduction ratio of the optical system. As a result, a mechanical construction is complex and there is a big difficulty in maintaining a precision for the transfer of a very fine pattern.
U.S. Pat. No. 3,698,808 discloses a projection exposure apparatus which includes a cata-dioptric system which projects a whole reticle image by an on-axis light beam by using a beam splitter in the projection optical system. In the known apparatus, there are many flares due to reflection in a refraction plane of an optical system downstream of the beam splitter, and no attention is paid to the characteristics of the beam splitter such as unevenness of a reflection factor, absorption and change of phase. As a result, a resolution power is so low that the apparatus cannot be used as a semiconductor manufacturing exposure apparatus. Further, a light utilization efficiency due to light loss by the beam splitter is as low as 25-10%.
Japanese Laid-Open Patent Appln. No. 2-66510 which corresponds to U.S. patent application Ser. No. 223,968 filed on Jul. 15, 1988, now U.S. Pat. No. 4,953,960, discloses an optical system which includes a cata-dioptric optical system which uses a beam splitter in the projection optical system. In this optical system, however, many flares exist as they do in the optical system described above, and the light loss is large and the resolution power is low because of the non-uniformity of the reflection factor at the beam splitter and the non-uniformity of the phase change to an angle of incidence caused by the absorption, characteristic and the multi-layer film. Accordingly, it also has a big problem for practical use.